Description of the Prior Art
In a battery driven thermal printer, in order to prevent the decrease of the record density as the battery voltage drops, a DC--DC converter is used to maintain a voltage applied to a recording head at a constant level or a battery which exhibits a relatively low voltage drop due to the reduction of a battery capacity (e.g. nickel-cadmium cell) is used. However, when the DC--DC converter is used, a large energy is consumed by the DC--DC converter per se and hence a utility efficiency of the battery is not improved. On the other hand, the nickel-cadmium cell is expensive.